1. Field of the Invention
The present invention relates generally to measurement of the partial pressure of dissolved gases in a liquid and, more particularly, is concerned with an instrument for measuring on-line the effective partial pressure of dissolved gases in a liquid.
2. Description of the Prior Art
It is important in many fields to be able to know accurately and quickly the total effective partial pressure of dissolved gases in a liquid media. One field in which this is the case is the design and manufacture of centrifugal pumps.
Centrifugal pumps have a characteristic called required Net Positive Suction Head (NPSHr) NPSHr is a function of the hydraulic design of the pump and represents the minimum required margin between pump suction pressure and the vapor pressure of the pumped liquid. Failure to provide sufficient NPSHr may unduly restrict operation of the system in which the pump is installed. This is particularly true of nuclear reactor systems which have temperature/pressure restrictions due to brittle fracture concerns. Therefore, it is important for pump manufacturers to provide accurate NPSHr data for their pumps.
Pump manufacturers determine NPSHr by testing the pump. In the test procedure, the pump flow is held constant and the static suction pressure is decreased until one of the following events occurs: either cavitation bubbles appear on the impeller vanes (performed only if a viewport is available to look directly at the impeller suction eye), or the pump developed head falls off a certain amount (normally zero percent, one percent, or three percent head fall-off is used as the criteria). Then, the following formula is applied to determine NPSHr:
NPSHr=Ps+Pq-Pv,
where
P.sub.s =the static suction pressure measured during the test. PA1 P.sub.q =the velocity head pressure at the impeller suction which is obtained using the flow measured during the test and the suction diameter of the impeller which is known prior to test. PA1 P.sub.v =the vapor pressure of the liquid being pumped.
The vapor pressure of the pumped liquid consists of the vapor pressure of the pure liquid (which is a function of temperature at the pump suction which is measured during testing) plus the effective vapor pressure of any dissolved gases.
The vapor pressure of the dissolved gases is normally determined using one of the following two prior art methods. A first method is to degas the test loop and assume that the partial pressure of the gases is zero or some fixed value. A second method is to obtain a sample of the pumped liquid in a bomb and analyze the sample for types of gases and content of each gas. Then, apply known laws of physics to determine partial pressure of the gases in the operating liquid.
The above prior art methods have significant disadvantages. The first method fails to achieve the desired degree of accuracy. The second method is unable to determine the gas pressure on-line. In addition, the second method also may not be as accurate as desired because while vaporization (cavitation) of a liquid is a thermodynamic process involving a change of phase, dissolved gases coming out of solution is a mass transport process which requires a considerable longer time span than the change of phase.
Consequently, a need exists for a way to accurately determine on-line the effective partial pressure of any dissolved gases in a liquid.